This invention relates to an improved supervision circuit in a subscriber line interface circuit (SLIC). A SLIC is the interface circuit between a local telephone company central office (exchange) and an individual telephone (the "local loop"). The SLIC is located at the central office. The basic requirements for an SLIC card are referred to as BORSCHT, an acronym for the following functions:
Battery--The SLIC must supply power to the subscriber telephone.
Over voltage protection--The SLIC must protect the central office circuitry against voltage transients.
Ringing--The SLIC must cause the subscriber telephone to ring.
Supervision--The SLIC must detect service requests (when the caller goes off-hook), detect dialing input signals, and supervise calls in progress (when a ring is answered or when one party hangs up).
Coding--The SLIC must convert the voice signal into serial digital codes that are placed into pulse code modulated time slots for digital transmission. A decoder is required at the other end of the transmission circuit. Both encoder and decoder are commonly combined into one integrated circuit called a codec.
Hybrid--The SLIC must convert a two-wire line from the subscriber telephone set to the four-wire line used for long distance communication.
Test--The SLIC must allow access for central office circuits to detect faults in the local loop.
Within a SLIC, a supervision circuit must provide certain functions. When an individual telephone is called (the answer mode), the central office, through the supervision circuit in the SLIC, provides a 20 Hz ringing signal to the called telephone. When the called party answers the telephone by lifting the receiver, the off-hook switch causes a switch to close in the telephone which allows a DC current from the battery supply at the central office to flow in the local loop. The central office detects that the telephone has been answered by this current flow. The supervision circuit must be able to detect the flow of DC current at the same time that the AC ringing current is present. The Bell Telephone and other company's standard requirement for such a circuit is that the DC current be detected sufficient quickly to enable the ringing signal to be removed within 200 milliseconds. This allows the ringing to be removed before the receiver is applied to the called party's ear.
The amount of the DC current in the local loop can vary widely. It depends on the central office exchange battery voltage, the current limiting resistance inserted into the loop at the exchange to limit short circuit current, the resistance of the local loop, and the resistance of the telephone set itself. Therefore the supervision circuit must be capable of detecting a wide range of DC currents but must not be so sensitive that leakage currents are detected as an off-hook condition. A loop threshold current is selected in light of these factors so that a DC current above the threshold is presumed to indicate an off-hook condition.
To detect the DC current in the presence of the 20 Hz ring signal, a low-pass filter is typically used to filter out the 20 Hz signal. The characteristics of the low-pass filter must be carefully chosen so that the change in DC current indicating an off-hook condition is detected within 200 milliseconds. This usually involves a compromise in the quality of the filter response.
When a call is initiated from a local telephone (calling mode), the supervision circuit must detect the off-hook condition and then detect the numbers entered. When dialing, a rotary telephone causes DC current to be interrupted by opening a switch, with the number of interruptions corresponding to the number dialed. The supervision circuit is required to detect these interruptions in the DC current due to dialing pulses. The mechanical inertia associated with a rotary dial telephone places an upper limit on the pulse rate of approximately 10-20 pulses per second (10-20 Hz). (Dual tone multi-frequency (DTMF) push button phones use the generation of alternating current tone frequencies, and not the interruption of DC current, and are interpreted by a separate circuit.)
The filter used to enable detection of an off-hook condition in the presence of the ringing signal inhibits the detection of dialing pulses because the dialing pulse rate can be very close to the 20 Hz ringing signal rate. Even where the pulse signals are not significantly attenuated by the filter, the rising and falling edges on the change in DC current are lengthened due to the presence of the filter. This degrades the detected dialing duty cycle and can result in errors. The degradation is aggravated by variations in line impedance which causes variations in the rise and fall times of the dialing pulse signals.